O-glycosylation of SARS-CoV-2 spike protein in insect and human cell lines Sites of O-glycosylation on the spike protein were near N-glycosites, suggesting O-glycans complement N-glycans' effect in immune shielding. The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of the COVID-19 pandemic, has some characteristics that were not seen before in human coronaviruses, such as mutation on the receptor-binding domain (RBD) for high affinity to the human angiotensin-converting enzyme 2 (ACE2) and presence of a furin cleavage site between the two subunits of the spike protein. Several vaccines have now been approved for use, and many more are under clinical trials. Some vaccines are subunit formulations that use recombinant viral proteins for immunization.